This application is the national phase under 35 U.S.C. xc2xa7371 of prior PCT International Application No. PCT/JP97/03224 which has an International filing date of Sep. 12, 1997 which designated the United States of America.
The present invention relates to an apparatus for spectrum spread communication using a CDMA mobile communication system, and particularly for a CDMA variable rate transmission method with stable performance for high-speed transmission and a variable rate transmission apparatus based on the method.
Active research and development have been carried out for third-generation mobile communication systems. In the next-generation third system, because multimedia communications are considered as main service, there has been a demand for the function of high capacity and for the function of data transmission of various rates with the required minimum transmission power on a flexible and high-quality basis. There is much attention on a multiple access system using a spectrum spread communication, i.e., a CDMA (Code Division Multiple Access) system as the radio access for this next generation radio.
Spectrum spread communications based on direct sequence are communications in which an information signal is multiplied by spreading which results in a transmission band width wider than the information signal band width. The spectrum spread communication has features such as low probability of intercept, anti-interference, anti-multipath fading, multiple access possibility, etc. The multiple accessing system means that a plurality of mobile stations communicate simultaneously with a base station. The performance of the spectrum spread communication depends on a spreading factor. The spreading factor indicates the ratio of the transmission band to the information signal band, i.e., the ratio between a spreading-code rate and an information transmission rate. One obtained by representing the spreading factor in decibel (dB) is called xe2x80x9cprocessing gainxe2x80x9d. When the information transmission rate is 10 kbps and the spreading-code rate is 1 Mcps (chips per second), for example, the spreading factor reaches 100 and the processing gain results in 20 dB.
As described above, the multiple access system using the spectrum spread communication is called xe2x80x9cCDMAxe2x80x9d. In the present CDMA system, different spreading codes are used in every user or channel to identify the users or channels.
It has been reported by Gillhauzen et. al from the following reference that the CDMA system is superior to other multiple access systems such as a TDMA (Time Division Multiple Access) system in channel capacity (the number of channels in the same band). The reference is as follows: xe2x80x9cOn the Capacity of a Cellular CDMA Systemxe2x80x9d, IEEE Transactions on Vehicular Technology vol. 40, No. 2, May.
In addition to this, the CDMA system has an advantage that since all the radio cells (radio zones) are allowed to use the same frequency, the CDMA system can relatively easily implement diversity handoff (or soft handoff) which are not handled well by the TDMA system. Further, in contrast with the TDMA system, multipath signals, which might degrade quality multi-path signals, are separated from others and identified by RAKE reception and can be effectively synthesized in reverse. Thus excellent transmission quality can be achieved with less transmitting power.
FIG. 1 is a block diagram showing an un-link transmission system employed in the conventional coherent multicode DS-CDMA (Direct Sequence CDMA). In the up-link transmission system, the length of one frame is 10 ms and where the user data and control data (c) are multiplexed on a time basis. In order to detect an error produced in the frame, a 16-bit CRC (Cyclic Redundancy Check) is used to perform error detector coding and a 6-bit tail bit (Tail) is added thereto. Thereafter, convolutional coding of a ⅓ rate, which is included into some of a spreading process, is performed. Since the error detecting process is completed for each frame, a structure applicable to packet transmission is adopted in the conventional example.
FIG. 2 is an explanatory view showing the insertion of pilot symbols, used for estimation of fading, into coded data (Coded Data) after having been interleaved in the conventional up-link transmission system shown in FIG. 1. In the drawing, (a) indicates the case in which a transmission rate (data rate) of transmit data is less than 32 kbps and (b) indicates the case in which the data rate is less than 128 kbps. As shown in FIG. 2, the transmit data or its sequence is divided into slots every 0.5 ms after the completion of bit interleaving. In the case of a 32(128) kbps code channel, 4(16)-bit pilots are inserted into the transmit data sequence, after which data modulation (QPSK) is performed (at this time, 2(8) pilot symbols corresponding to 2(8) symbols are provided). Hence spreading modulation is done by dual spreading codes. In the conventional example, an orthogonal gold sequence is used as a short spreading code and a gold sequence is used as a long spreading code. Further, BPSK (down-link) and OQPSK (up-link) are used for the spreading modulation.
FIG. 3 is an explanatory view showing the insertion of pilot symbols used in coherent multicode multiplex transmission in the conventional up-link transmission system shown in FIG. 1. In the drawing, (a) indicates the case in which a data rate is lower than a predetermined rate, e.g., 32(128) kbps, and (b) indicates the case in which the data rate is higher than 32(128) kbps. Upon the transmission of data at a fast rate (when the data rate is greater than 32/128 kbps), a transmission data sequence is subjected to error correction coding and bit interleaving and thereafter divided into a plurality of code channels, which in turn are respectively subjected to data modulation and spreading modulation separately. In this case, concatenated coding is applied in which convolutional codes each having a rate ⅓ are used as inner codes and Reed Solomon codes RS(40, 34) with one symbol as 8 bits are used as outer codes. Since a propagation path is common between all the code channels, each fading-estimated pilot symbol is inserted only into a first code channel in the case of an up-link as shown in FIG. 3.
The multicode multiplexed CDMA system typified by the up-link transmission system of the conventional coherent multicode DS-CDMA (Direct Sequence CDMA) above has a problem in that when the data rate of each transmit signal reaches high rates, it is difficult to maintain the linear characteristic of the power amplifier, with the result that the amount of interference in the adjacent frequency bands increases. Namely, in a communication device of a conventional multicode multiple CDMA system, the number of multicode multiplexings increases as the data rate of the transmitted signal increases, so that the degree of a envelope variation increases after the multiplexing. The power amplifier used for power amplification has a problem in that it normally performs power amplification faithfully with respect to an amplitude variation lying within a predetermined range (linear region). However when the width of the amplitude variation exceeds a limit, an input-to-output linear characteristic cannot be kept, so that distortion caused by a nonlinear characteristic results in an increase in the amount of interference with adjacent frequency bands.
The present invention has been made to solve the above problems. It is therefore an object of this invention to obtain a variable rate transmitting method capable of holding a linear characteristic of a power amplifier and providing high-quality data transmission in a simple hardware configuration, and a variable rate transmission apparatus using the variable rate transmission method.
An apparatus for variable rate transmission according to the present invention comprises means for transmission spreading modulation on data signals in a binary sequence using bi-orthogonal signals when a transmission rate of each data signal is greater than or equal to a predetermined transmission rate.
Owing to this arrangement, an advantageous effect is brought about in that even in the case of a fast data rate, a linear characteristic of a power amplifier can be maintained and high-quality data transmission can be implemented in a simple hardware configuration without giving interference to adjacent frequency bands.
The variable rate transmission apparatus according to the present invention further comprises signal processing means for effecting a series of signal processes such as an error correction coding process on the data signals, and a first serial/parallel converter for performing serial/parallel conversion on the output of the signal processing means. The means for transmitting spreading-modulated data signals in a binary sequence using bi-orthogonal signals spreading-modulates parallel output signals outputted from the first serial/parallel converter in a binary sequence using the bi-orthogonal signals and transmits the resultant signals therefrom.
Owing to this arrangement, an advantageous effect is brought about in that even in the case of a fast data rate, a linear characteristic of an output produced from a power amplifier can be maintained and high-quality data transmission can be implemented in a simple hardware configuration without giving interference to adjacent frequency bands.
The variable rate transmission apparatus according to the present invention further comprises a second serial/parallel converter for converting data signals into serial/parallel form, and signal processing means provided so as to correspond to the respective parallel data signals outputted from the second serial/parallel converter and for performing a series of signal processes such as predetermined error correction coding on the signals. The means for spreading modulation on the data signals in the binary sequence using bi-orthogonal signals and for transmission of the spreading-modulated signals spreading-modulates signals outputted from the signal processing means and transmits the resultant signals therefrom.
Owing to this arrangement, an advantageous effect is brought about in that even in the case of a fast data rate, a series of signal processing rates can be all achieved with the same rate and the hardware design can be easily carried out, and the linear characteristic of a power amplifier can be maintained and high-quality data transmission can be implemented in a simple hardware configuration without interfering with adjacent frequency bands.
The variable rate transmission apparatus according to the present invention is constructed such that the means for spreading-modulation on the data signals in the binary sequence using the bi-orthogonal signals and transmission for the spreading-modulated signals generated from the bi-orthogonal signals using Walsh functions.
Owing to this arrangement, an advantageous effect is brought about in that bi-orthogonal signals are easily generated, transmitted and detected, and high-quality data transmission can be carried out.
A variable rate transmission method according to the present invention is constructed such that when a transmission rate of each data signal is greater than or equal to a predetermined transmission rate, data signals are spreading-modulated in a binary sequence by using bi-orthogonal signals and are transmitted.
Owing to this arrangement, an advantageous effect is brought about in that even in the case of a fast data rate, a linear characteristic of a power amplifier can be held and high-quality data transmission can be implemented without giving interference to adjacent frequency bands.
The variable rate transmission method according to the present invention is constructed such that Walsh functions are used to obtain bi-orthogonal signals.
Owing to such an arrangement, an advantageous effect is brought about in that bi-orthogonal signals can be easily generated, transmitted and detected.